Various metals, including gold and other precious metals, naturally occur in ores in several different forms and complexes. Some metals are recovered from their mined ores by the use of aqueous solutions to leach or dissolve the desired metal from such ore. The aqueous solution used for a given metal contains some chemical agent that converts the metal to a soluble form. For instance, gold may be recovered from a gold-containing ore by leaching such ores with an aqueous solution of cyanide, typically but not necessarily sodium cyanide to form gold cyanide. Such a species is soluble in water or at least in the aqueous leaching solution at a concentration sufficient for pregnant liquors.
During direct cyanidation, gold-bearing ore is hence crushed, ground and added to a lime and sodium cyanide solution before it thickens. The mixture is left in agitators for a period of 24 to 48 hours. Once gold has dissolved, it is recovered using activated carbon, resin or precipitated powdered zinc.
World wide, there exists many ores that are considered refractory to conventional extraction processes. For example, in refractory gold ores, the gold is typically finely disseminated in a pyretic mineral and cannot be treated by conventional methods. Indeed, gold may be physically entrapped in a matrix of sulphide metals, such as iron pyrite and arsenopyrite. The matrix is not penetrated by cyanide solutions and, thus, a cyanide solution alone cannot extract and dissolve the gold from such ore.
If the conventional direct cyanidation method is applied to arsenopyrite ore, or other mineral assemblages refractory to cyaniding, only 40 to 60 percent of the gold eventually dissolves.
Accordingly, the prior art has shown examples of methods used for extracting so-called refractory ores. One such method involves first reducing the refractory ore to a concentrate. The latter is then roasted prior to being cyanidated to liberate gold by evaporation of sulphides and arsenic. However, this method presents serious environmental drawbacks and, as such, is even prohibited in some countries such as Canada, for example.
Other methods involve concentrating the refractory mineral ore and processing the concentrate using various methods. For example, the concentrate may be sent to a copper refinery where gold is recovered and sulphides are used as combustible matter. However, the ore must have a very low arsenic content, otherwise refineries will refuse to process the concentrate. Alternatively, the concentrate may be leached by autoclaving in ore to dissolve all of the sulphide and arsenic particles prior to cyaniding gold. This method is, however, very expensive to install and operate. Furthermore, it generates by-products that are potentially very harmful to the environment. In, yet, another alternative method, concentrates undergo bacterial lixiviation in order to render sulphide crystals porous by dissolving part of the sulphide and arsenic before cyaniding gold. This method is also slow and expensive to operate and relatively unstable since bacteria can be compromised by slight temperature variations. Furthermore, it produces by-products that, again, may prove to be highly detrimental to the environment.
Accordingly, there exists a need for an improved method and device for extracting refractory mineral ores such as refractory precious metal ores including refractory gold-bearing ore. It is a general object of the present invention to provide such an improved extraction method and device.